I only use Windows to develop a single application and for no other reason, so this is a wild guess. I've written a lot of RS232 along with other serial communication software, but not for this platform...

However, if I cross to a write then read program, coping the above RealTerm success of outputting "ati\n", I get no success.

What if you try ati\r\n?

Maybe on Windows the carriage return makes a difference?